1. Field of the Invention
The present invention relates to a light-emitting zinc-oxide-based compound semiconductor device and, more particularly, to a light-emitting compound semiconductor device having a double heterostructure that is capable of emitting high-power visible light ranging from ultraviolet to red, as desired, by changing the composition of a compound semiconductor constituting an active light-emitting layer.
2. Description of the Related Art
For some time, there has been interest in producing II-VI compound wide band gap semiconductors to produce green/blue light-emitting diodes (LEDs), laser diodes (LDs) and other electrical devices. Historically, attempts to produce these devices have centered around zinc selenide (ZnSe) or gallium nitride (GaN) based technologies. However, these approaches have not been entirely satisfactory due to the short lifetime of light emission that results from defects, and defect migration in these devices.
Recently, because zinc oxide (ZnO) has a wide direct bandgap of 3.3 electron-Volts (eV) at room temperature and provides a strong emission source of ultraviolet light, ZnO thin films on suitable supporting substrates have been proposed as new materials for LEDs and LDs. Undoped, as well as doped, ZnO films generally show n-type conduction. Impurities such as aluminum and gallium in ZnO films have been studied by Hiramatsu et al. who report activity as n-type donors (Transparent Conduction Zinc Oxide Thin Films Prepared by XeCl Excimer Laser Ablation, J. Vac. Sci. Technol. A 16(2), March/April 1998). Although n-type ZnO films have been available for some time, the growth of p-type ZnO films necessary to build many electrical devices requiring p-n junctions has been much slower in developing.
Minegishi et al. (Growth of P-Type ZnO Films by Chemical Vapor Deposition, Jpn. J. Appl. Phys. Vol. 36 Pt. 2, No. 11A (1997)) recently reported on the growth of nitrogen doped ZnO films by chemical vapor deposition and on the p-type conduction of ZnO films at room temperature. Minegishi et al. disclose the growth of p-type ZnO films on a sapphire substrate by the simultaneous addition of ammonia (NH3) in carrier hydrogen and excess Zn in source ZnO powder. When a Zn/ZnO ratio of 10 mol % was used, secondary ion mass spectrometry (SIMS) confirmed the incorporation of nitrogen into the ZnO film, although the nitrogen concentration was not precisely confirmed. Although the films prepared by Minegishi et al. using a Zn/ZnO ratio of 10 mol % appear to incorporate a small amount of nitrogen into the ZnO film and convert the conduction to p-type, the resistivity of these films is too high for application in devices such as LEDs or LDs. Also, Minegishi et al. report that the carrier density for the holes is 1.5×1016 holes per cubic centimeter (holes/cm3), which is considered to be too low for use in commercial LEDs or LDs.
Park et al. in U.S. Pat. No. 5,574,296 disclose a method of producing thin films on substrates by doping IIB-VIA semiconductors with group VA free radicals for use in electromagnetic radiation transducers. Specifically, Park et al. describe ZnSe epitaxial thin films doped with nitrogen or oxygen wherein ZnSe thin layers are grown on a GaAs substrate by molecular beam epitaxy. The doping of nitrogen or oxygen is accomplished through the use of free radical source which is incorporated into the molecular beam epitaxy system. Using nitrogen as the p-type dopant, net acceptor densities up to 4.9×1017 acceptors per cubic centimeter (acceptors/cm3) and resistivities less than 15 ohm-cm were measured in the ZeSe film. However, the net acceptor density is too low and the resistivity is too high for use in commercial devices such as LEDs, LDs, and field-effect transistors (FETs).
White et al in U.S. Pat. No. 6,291,085 disclose a method for producing ZnO films containing p-type dopants, in which the p-type dopant is arsenic and the substrate is gallium arsenide (GaAs). The method of preparation of the film is laser ablation. However, the crystal quality of the films prepared by such a process is inferior and not suitable for device applications.
Hideo et al in US Patent Publication No. 20030132449 disclose a method for developing an ultraviolet-light-emitting semiconductor diode comprising an n-type ZnO layer with luminous characteristics formed on a transparent substrate, and a p-type semiconductor layer selected from the group consisting of strontium copper oxide (SrCu2O2), copper aluminum oxide (CuAlO2) and copper gallium oxide (CuGaO2), which is formed on the n-type ZnO layer to provide a p-n junction therebetween. However, device reliability of these hetero structures is less than desired.
Although some progress has recently been made in the fabrication of p-type doped ZnO films which can be utilized in the formation of p-n junctions, a need still exists in the industry for ZnO films which contain higher optical output and produce a reliable device.